Turret for drilling or production ship

ABSTRACT

A turret for a vessel, such as a drilling or production vessel for recovery of oil offshore, is installed in such a manner that it can rotate in a throughgoing opening or well in the hull of the vessel, and includes bearing arms which are equipped with axially and radially fitted bearing elements which act against corresponding bearing elements on the vessel. The bearing arms are connected to a substructure in the turret which provide individual suspension to and can absorb unevennesses and deformations in the bearing. A track of the axial bearing is disposed on a pedestal-like elevated area that is rigid in the axial direction. The pedestal-like elevated area is connected with the hull, mainly at the level of the neutral axis of the vessel, and the radial bearing element on the vessel is in the form of a band-like structure.

BACKGROUND OF THE INVENTION

The present invention relates to a turret for a vessel such as adrilling or production vessel for recovery of oil offshore, such turretbeing erected so as to allow rotation in a throughgoing opening or wellin the hull of the vessel, and having suspension arms which are equippedwith axially and radially provided bearing elements which operate inrelation to corresponding bearing elements on the vessel.

A turret of the abovementioned type is normally fitted with bearingelements with spring devices to assure an even distribution of thebearing forces. The suspension arrangements of the bearings have afairly large slack, partly to absorb elongation in the vessel, and areoften jointed to handle angular deformation and to even out loads. Inorder to achieve the best possible control of suspension forces anddeformation in the bearings, the vessel and rotary or tower turret,complicated mechanical or hydraulic solutions are often used. Ahydraulic solution is shown in EP Patent Application No. 0,207,915. Itconsists of an upper radial bearing, an axial bearing and a lower radialbearing. Each of these bearings consists of a large number of hydraulicpiston/cylinder devices which are each mounted on a bearing element.

One major disadvantage with these solutions is that they arecomplicated, and therefore expensive to build and maintain. A furtherdisadvantage is that the bearing surfaces are subject to wear as aresult of relative movements and constructional deviations which are dueto the suspension/bearing wheel arrangement and to movements in thevessel. With regard to the wheel arrangement, because of large relativemovements pommelled on curved surface wheels have to be used. Thesepommelled wheels have limited bearing capacity, and in the case oflarge, heavy rotary bearing structures, slide bearings therefore have tobe used, or a combination of wheel and slide bearings.

One disadvantage with slide bearings, however, is that large machineryis required to turn the turret, and special, expensive precautions haveto be taken to protect the bearings against the corrosive environment onboard vessels at sea.

Norwegian Patent No. 165,285 shows a bearing system for a turret inwhich an attempt is made to eliminate the wear and tear on the radialbearing by using a structural suspension. However, this structuralsuspension has limited independent suspension, particularly in the caseof large, heavy turret, which is necessary to maintain a satisfactoryload distribution without using special mechanical or hydraulic springsin connection with the axial bearing elements. The wear and tear on theradial bearing surfaces is thus not quite eliminated by this solution.

Further, with regard to the aforementioned Norwegian patent, mechanicalsuspension is also used in the radial bearing of the turret, and thissuspension is, as previously mentioned, costly to build and maintain,and will cause wear and tear on the axial bearing.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a turret for vesselsin which the wear and tear on the axial and radial bearings of theturret is virtually eliminated, but which is nevertheless cheaper tobuild and maintain than existing solutions. Another object is to providea bearing design for such turret in which vessel-induced stresses andelongations do not induce undesired reaction forces on the bearing andthe rotary tower on turret. A third object is to reduce displacements inthe turret due to external forces which act thereon. A fourth object isto provide a turret solution in which unevennesses in bearing tracksetc. are absorbed by the substructure of the rotary tower and/or thebearing tracks themselves. Last, but not least, one major object is toprovide at a solution which can be used on large, heavy rotary towerswhich are subject to large forces.

In accordance with the invention, there is provided a turret whereinbearing arms are connected with a substructure in the turret whichpermits the bearing arms to individually absorb irregularities in thebearing surfaces, the foundation for the bearings for the turret isdisposed basically on a level with the neutral axis of the ship, theaxial bearing is disposed on a pedestal which is rigid in the axialdirection, and is designed to absorb displacements in the radialdirection.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of example andwith reference to the accompanying where:

FIG. 1 is a schematic longitudinal section of a turret according to theinvention and shown installed in a vessel.

FIG. 2 partial perspective view of substructure of the turret shown inFIG. 1.

FIG. 3 is a section at a larger scale through a bearing device for theturret.

FIG. 4 is an end view of the bearing device as seen from above.

FIGS. 5 and 6 are views similar to FIG. 3 and show two alternativebearing designs.

FIG. 7 is a schematic view showing principles of a ballast system forthe turret.

DETAILED DESCRIPTION OF THE INVENTION

The turret shown in FIG. 1 is mounted in a throughgoing opening or well3 in a hull 2 of a vessel. A lower part 4 of the turret, i.e. asubstructure thereof, includes a substantially cylinder-shapedstructure, while an upper part 1 of i.e. a turret, the manifold chamberdefining portion, includes a circular deck 5 which provides space forpipe systems and equipment. Oil and gas rises 9 are led through guidepipes 19 up to a choke and manifold system (not shown). A swivelcoupling 20 with a set of pipes 21 connects the flow of produced oil andgas from the turret to process equipment of the vessel via a framestructure 22.

The vessel may be dynamically positioned or anchored via mooring linesconnected with the turret. In the example shown here, mooring lines 8are led via a guide wheel 11 on the outside of the turret and areattached to stoppers 12 which are fitted inside the top of the turret.Mooring line lifters 13 mounted on the deck or winches (not shown)mounted on the turret are used to tighten the mooring lines over a guidewheel 10. Alternatively, chain-stoppers instead for a guide wheel 11 canbe provided at the lower part of the turret. The guide wheels/chainstoppers 11 should preferably be mounted high (in relation to the baseline of the vessel) to reduce capsizing moments due to line elongationand to simplify docking of the vessel.

Turret bearings 28, 31 are arranged in an extended upper part of thewell 3 along the neutral axis of the vessel. By arranging the bearingsmainly on a level with the neutral axis of the vessel, hull-inducedmovement in surfaces of the bearings is reduced. A capsizing effect isalso reduced, i.e. the distance between the bearings 28, 31 and theguide wheels 11 will be as short as possible.

The turret can be rotated by means of cable lifters 13 via drive chains(not shown in detail) arranged along the circumference of the turret, ora separately rigged rotary device can be used which includes a gear 24driven by a motor 23. The gear 24 engages with a toothed wheel rim 6 onthe turret.

The lower part 4 of the turret includes a solid, ring-shaped box-likesupport base 35 that forms a foundation for the guide wheel 11 of themooring lines. Support base 35 has a chamber 34 which preferably may bedivided into separate tanks by radial bulkheads. With the aid of aballast system (discussed below such tanks can be filled or emptied asdesired (depending on stretch in the mooring lines) to reduce thecapsizing moment of the turret.

FIG. 2 shows that the substructure 4 of the turret includes supports orbearers 16, radial arms 15, a basically cylindrical column 36, thering-shaped box-like support base 35 and a top plate 32. The radial arms15 are fastened to respective vertical bearers 16, which in turn areconnected to the base support 35. In FIG. 2, the vertical bearers areshown to be T-shaped, but they can with advantage be H-shaped,box-shaped or other appropriate shapes.

The cylindrical column or plate structure 36 between the verticalbearers 16 is largely shear-rigid in a vertical plane, but preferably isflexible in radial directions.

The top plate 32 is shear-rigid, and can be reinforced with a flangering 29 or similas structure in order to achieve adequate radialrigidity. Apart from that, plate 32 is mainly stiff in the horizontalplane, but preferably is flexible laterally thereof. Plate 32 is alsoprovided with openings 33 for the riser, guide pipes 19 (see FIG. 1).

As mentioned above, each of the radial arms 15 is fixed to a respectivevertical bearer or support 16. Moment loading induced by an arm 15 willcause rotation at the point where such arm is fixed, and respectivevertical bearer 16 will deflect without affecting adjacent bearers. Thisis possible because the structures (plate/stiffeners) between thevertical bearers have an insignificant stiffness to deformation in theradial direction of the turret.

With the substructure described above for the turret, one achieves anindependent, structural suspension for each of the arms 15, which isnecessary to absorb unevennesses in bearing tracks of the bearings. Withstructural suspension, it will also be advantageous to use wheels in theradial bearing, since major cross-movements of the bearing elements willbe avoided in case of large loads. Use of wheels in the axial bearing(also the radial bearing) also reduces rotary moments when the turretrotates.

The proposed substructure thus represents a second important feature ofthe invention, since it is substantially cheaper than the knownsolutions which, as mentioned above, use hydraulic or mechanicalsuspension to absorb the same unevennesses. In this connection it shouldbe mentioned that in Norwegian Patent No. 165,285 a turret with radialarms is used, but such arms are connected to a torque box. This torquebox provides flexibility against axial loads which act on the wholeturret, since all the arms are fixed to a common box structure. But sucharrangement does not contribute substantially to independent deflectionwhich is necessary to absorb unevennesses in the bearing race.

FIGS. 3 and 4 show on enlarged scale the bearing arrangement of theturret. As mentioned previously, the bearing arrangement is largelyaligned with the neutral axis of the vessel to reduce hull-inducedmovements and loads on the bearings.

The bearing arrangement includes radial wheel bearing 28 and axialbearing 31. Bogies 40 attached to each of the arms 15 of the turret areused for the axial bearing. Each bogie includes bogie wheel pairs 41fastened to each end of a tangential girder 42. Girders 42 are providedwith a wide, lower flange or shear plate 43 which is rigid to radialloads from the radial arm 15. The tangential girders are designed mainlyto be rigid to loads in the axial direction, but to allow rotation inrelation to the radial arm 15. This assures that the tangential girdersare rigid to radial and axial deformations, but nevertheless allow anevening out of the load between the four wheels 45 in the bogie. It isimportant that the girder 42 is flexible enough to tolerate downwardbending of the arm 15 without this producing too great reaction loads inthe wheels 45. Alternatively, the arms can be built with a certainpre-load angle which is opposite to the downward bending when the turretis subject to maximum loading, the object being that the loads on thewheels are as even as possible when the rotary tower or turret issubject to extreme loads.

The wheels 45 are mounted in a shear-rigid frame 46, so that the wheelsare rigid in relation to one another. The wheels 45 can therefore toadvantage be made with cylindrical surfaces. A slide bearing shouldpreferably be used in the hub of the wheels to achieve a suitableresistance to rolling and at the same time allow the wheels to slideaxially along its axis, in order to absorb relative, radial deformationsbetween the radial bearing and the axial bearing, and to absorbdeviations due to construction between the position of the radialbearing and the rails.

Using wheels with cylindrical surfaces has the advantage over curved orpommelled wheels that they have a considerably greater bearing capacity.

The bogie 40 for the axial bearing rolls on a double rail system ofrails 44 and the rails in turn rest on a pedestal-like foundationconsisting of two cylindrical columns 30, and a ring-shaped or annulartorque box 47. Between the box 47, the columns 30 with the necessarybracing and the deck 48, there is no structure which would allow the twoshells or columns 30 to be freely deformed in a radial direction. Theupper torque box can also be regarded as an upper rigid ring whichensures that the bearing tracks retain their shape locally in the radialplane, while the columns absorb global relative displacements betweenthe bearing tracks and deck support. The plate structure 36 has openings37 designed to allow air to pass through.

The columns are rigidly supported in the structure of the vessel, well 3and a support in the deck of the vessel respectively, so that the axialposition of the two rails 44 in principle remains at the same elevationwhen the hull of the vessel is subjected to loads and elongations.

One major advantage with the present foundation design is thus thatradial elongations in the hull of the vessel are filtered out by meansof a radially flexible spacer, i.e. the columns 30, between the deck 48of the ship and the bearing tracks 44. This substantially reduces wearon the surfaces of the bearing compared with known solutions. In orderto further reduce wear and tear and possibly to increase suspension ofthe bogies 40 in relation to arms 15, rubber fillers 26 may be insertedbetween the bogies 40 and the arms 15. These fillers will also eliminatesliding movements in the bearings of the wheels in the bogies 40, andwill help to even out the load on the wheels 45.

The radial bearing includes wheels 49 fitted close together in a rim 50which is connected with the radial arms 15. The wheels 49 run against aradial bearing rail 51 which is fixed to a cylindrical band 52. The band52 and the rail 51 have a substantial tangential tensile strength, buthave local flexibility to minor deviations in the establishment of themutual radial position of the rail 51 and the wheels. As regards theradial position of the wheels, this can be secured by means of wedgedevices 27 which move the wheels in or out in relation to the rim 50, ora kind of cam axle arrangement can be used.

The upper band 52 includes a column shell which extends from the loweredge of the rail up and a bit past top of the rail. This breadth isdetermined by the necessary tangential strength and radial flexibilityof the rail. The band can be strengthened with extra ring-shaped bracers53 which are placed a certain distance from the rail 51. The foundationfor the radial bearing shown includes a column which is an extension ofthe band 52 down to the deck 48. It can to advantage be made of a thinshell plate 54.

The radial load from the closely mounted radial wheels 49 is transferredto the rail 51/band 52 as tangential forces around the bearing band.Elongation in the band is transferred to the deck of the ship in therange of 45-135 degrees in relation to the load direction, via the lowerplate 54. Radial displacement of the turret is therefore limited.

The wheels 49 are mounted in rigid rim 50 on the turret, while thebearing band 51 must be sufficiently flexible to compensate for defectsin the rail and wheels. The wheels have to be mounted so close togetherthat limited flexion is caused in the rail/band section.

The advantage of a radial bearing design of the above described type isthat the band has enough structural suspension to compensate for localtolerances (unevennesses) in the rail and wheel mounting. Theovalisation of the deck around the well is absorbed in the foundation 54and/or by means of a certain clearance between rail 51 and wheels 49, sothat the radial bearing is maximally loaded as a result of theelongations of the vessel in heavy seas.

The band 52 and the plate of foundation 54 are also in principle soflexible in relation to radial deformations, that global ovalisations(defects) in the turret do not affect the bearing reaction forces to anysignificant degree.

Alternatively, the band 52 and the foundation 54 may be connectedtogether by means of a coupling 55. The purpose of this coupling is togive the column limited supplemental flexibility in relation to thedeck, whereby radial deformations of the well 3 reduce forces in theradial bearing, and that reaction forces in the radial bearing should beless affected by an ovalised turret.

FIG. 5 shows an example of an alternative design, where a box structure56 for the axial bearing is provided directly on the deck 48, i.e.without a flexible connection between the deck and the box structure.With this solution, a somewhat lower building height is obtained for thesubstructure, but there will be somewhat more wear and tear on thebearing surfaces.

FIG. 6 shows a further bearing solution in which the axial bearing andthe radial bearing are provided on a common pedestal 57, and in which abox structure 58 provides support for both rails 59 of the verticalbearing and rail 60 of the radial bearing. An internal plate 61 acts ina manner similar to the band (52, FIG. 3) mentioned above, since it isdesigned to compensate for minor uneveness in the wheels and rail, i.e.plate 61 is not braced.

The difference between this bearing and the bearing shown in FIGS. 3 and4 is that a separate column for the radial bearing is eliminated, andthe radial bearing is provided at a lower level, which helps to make acapsizing moment from horizontal forces which act on the turret smaller,and less steel for a foundation is needed.

FIG. 7 is a sketch showing the principles of the ballast system for theturret, according to the invention. The lower part of the turretconsists, as mentioned previously, of a solid, ring-shaped box-likesupport base 35 which can be divided into separate tanks 6, 7 in thecircumferential direction of the turret. With the aid of a pump systemand pipelines 61 between the tanks, ballast can be pumped from one ormore tanks on one side to one or more tanks on the opposite side toreduce the loads on the bearings and to reduce the capsizing moment ofthe turret. The pumps can to advantage be controlled by an electroniccontrol unit based on signals from tension detectors 14 on the mooringlines.

The above refers to an example of a turret solution in which wheelmountings are used for both the axial and the radial bearings. However,within the scope of the invention, as defined in the claims, slidebearings can also be used, or a combination of slide and rollerbearings.

I claim:
 1. In a turret installed in a well in the hull of a vessel suchthat said turret is rotatable relative thereto, said turret includingplural bearing arms supporting axial and radial bearing elements actingon corresponding respective axial and radial bearing track elements onsaid vessel, the improvement comprising:said bearing arms beingconnected to and supported on a substructure of said turret such thateach said bearing arm may be flexed individually and independently ofadjacent said bearing arms, thereby enabling said bearing arms toindividually absorb unevenness and deformations in said bearingelements; said axial bearing track element being mounted on an elevatedarea of a pedestal structure mounted on said hull of said vessel, saidpedestal structure being rigid axially of said turret; and said radialbearing track element comprising a cylindrical band-shaped member. 2.The improvement claimed in claim 1, wherein said pedestal structure ismounted on and extends upwardly from a deck of said vessel.
 3. Theimprovement claimed in claim 1, wherein said elevated area of saidpedestal structure is at a level of the neutral axis of said vessel. 4.The improvement claimed in claim 1, wherein said pedestal structureincludes an annular rigid torque box.
 5. The improvement claimed inclaim 4, wherein said torque box is mounted directly on a deck of saidvessel.
 6. The improvement claimed in claim 4, wherein said torque boxis connected to a deck of said vessel by at least one annular columnextending upwardly from a deck of said vessel.
 7. The improvementclaimed in claim 6, comprising two said columns spaced radially of eachother.
 8. The improvement claimed in claim 7, wherein said band-shapedmember comprises a radially inner wall of said torque box.
 9. Theimprovement claimed in claim 1, wherein said band-shaped membercomprises an annular column connected with a deck of said vessel by afoundation in the form of a thin shell plate.
 10. The improvementclaimed in claim 9, wherein said column and said foundation areconnected by a coupling.
 11. The improvement claimed in claim 9, whereinsaid column is reinforced by ring-shaped braces.
 12. The improvementclaimed in claim 1, wherein both said axial and radial bearing compriseroller bearings.
 13. The improvement claimed in claim 1, wherein eachsaid bearing arm has mounted thereon a bogie, said axial bearing trackelement comprises two parallel rails, and each said axial bearingelement comprises paris of roller bearings mounted on oppositecircumferential ends of a respective said bogie and running onrespective said rails.
 14. The improvement claimed in claim 1, whereinsaid radial bearing elements comprise roller bearings mounted closetogether on a rigid rim connected to said bearing arms.
 15. Theimprovement claimed in claim 1, wherein said substructure comprises alower ring-shaped support base, a cylindrical column extending upwardlyfrom said support base, plural vertical supports extending upwardly fromsaid support base within said column, said bearing arms being connectedto respective said vertical supports, and an upper top plate.
 16. Theimprovement claimed in claim 15, wherein said support base has therein achamber divided by bulkheads into separate ballast tanks.